Ben's answer above is good. I wish I could give him more than just one upclick.
Rules of thumb:
Carbs and proteins run 3.5 to 4 calories per gram. Fats run aobut 9 calories per gram.
Working hard, especially in cold weather, you can tolerate a lot of fat in your diet.
When planning food for teenagers, I figured on 4000 calories per day.
This is sufficient for days with 8-9 working hours per day. (Pretty hard core compared to most recreational use.)
With a 40% fat content diet, each hundred grams of food with provide 40 * 9 + 60 * 4 = 360 + 240 = 600 calories. So it would take 700 grams of food per day -- about a pound and a half. Our meals tended to be lower fat than that, and a rough rule of thumb was 2 lbs dry weight per person per day. This allowed for things like cheese and peanut butter which have moderate water content, but also have fats.
Two pounds per day means that the groceries for a 15 day trip are 30 lbs. With the high fat option (LOTS of nuts, lots of cooking oil) There really isn't getting around that.
So, as others have pointed out, you need to make the rest of your gear light, and minimize the parasitic weight (packageing) of your food.
If you do trips frequently, set up your food in a spread sheet. I had one in which I figured on the weight/volume per serving, had a constant for the number of people in the trip, another constant for the class of trip, and the spread sheet would figure out the packing weight/volume for everything. This makes things a BUNCH easier when packing for an expedition of 30 people for 3 weeks in the wild waters of northern Saskatchewan.
One aside: An external frame pack is considerably easier when handling lots of weight. They tend to be wider and flatter, so keep the load closer to your own centre of mass. They are however a true PITA in brushy country, as the extra width and exposed corners catch.
In response to a request, here is the link to my planning spreadsheet:
GREEN cells are filled in by formula.
The sheet has 4 tabs:
- Gear (I can provide, is stuff I have extra of that I was able to rustle up for my nephew)
You MUST be consistent in the names used on the Menu and Ingrediants tabs, or the VLOOKUPs don't work.
If you want to change the quanties, use the Ingrediants tab and change the serving size.
You can copy and modify.
Scenario: My nephew was flying out from the east to do a trip with me. He was 19, fit and about 140 pounds. I am on the wrong size of 65, not as fit, and was 175 pounds. Our packs were 40 pounds each.
Willmore wilderness is variable in terrain and climate. Snow can happen any month of the year. Trails are mostly horse trails. No bridges. Lots of stream crossings below timberline, and bogs in passes. You just live with wet feet. Our route was about 1/3 half century old logging roads, a third horse trail, and a third bushwhacking, and time above tree line. We covered about 120 km in 6 days.
We divided our lunch into two lunches, typically about 3 hours apart. We got onthe trail around 8, would have first lunch at 10:30 to 11, and second lunch around 2:00. We would camp between 6 and 7. This year there were heavy fires in B.C. and the sunlight ranged from yellow to orange. At times views were lost in the smoke under a mile away.
I mention this to give context to the gear list on the linked spreadsheet, and so that you can adjust quantities for your activity level.